Program controlled dicing of a substrate using a pulsed laser beam

ABSTRACT

A substrate is diced using a program-controlled pulsed laser beam apparatus having an associated memory for storing a laser cutting strategy file. The file contains selected combinations of pulse rate Δt, pulse energy density E and pulse spatial overlap to machine a single layer or different types of material in different layers of the substrate while restricting damage to the layers and maximising machining rate to produce die having predetermined die strength and yield. The file also contains data relating to the number of scans necessary using a selected combination to cut through a corresponding layer. The substrate is diced using the selected combinations. Gas handling equipment for inert or active gas may be provided for preventing or inducing chemical reactions at the substrate prior to, during or after dicing.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/511,743, which is a national stage application of InternationalApplication No. PCT/EP03/04069, filed Apr. 17, 2003, which claimspriority to Irish Patent Application No. S2002/0289, filed Apr. 19, 2002and Great Britain Patent Application No. 0225033.0, filed Oct. 28, 2002.

TECHNICAL FIELD

The invention relates to program-controlled dicing of a substrate,particularly, but not limited to, a semiconductor substrate, using apulsed laser.

BACKGROUND INFORMATION

Wafer dicing is a critical aspect of package assembly that facilitatesall subsequent operations in an assembly process. Wafer dicing isconventionally achieved by dicing a wafer with a mechanical saw. Use ofa mechanical saw has disadvantages such as low yield, chipping andcracking. Thin wafers cannot be machined due to the stresses induced inthe wafer by the saw resulting in low die strength. The strength of thedies produced when a semiconductor substrate is diced is an importantfactor as low die strengths reduce reliability. Improving die strengthminimizes breakages and the onset of micro-cracking and improves devicereliability.

SUMMARY OF THE DISCLOSURE

According to a first aspect of the invention, there is provided a methodof using a pulsed laser for program-controlled dicing of a substratecomprising at least one layer, the method comprising the steps of:providing program control means and associated data storage means forcontrolling the pulsed laser, providing in the associated data storagemeans a laser cutting strategy file of at least one selected combinationof pulse rate, pulse energy and pulse spatial overlap of pulses producedby the laser at the substrate to restrict damage to the respective atleast one layer while maximizing machining rate for the at least onelayer, providing in the laser cutting strategy file data representativeof at least one selected plurality of scans of the respective at leastone layer by the pulsed laser necessary to cut through the respective atleast one layer when the pulsed laser is operating according to therespective at least one combination stored in the laser cutting strategyfile; and using the laser under control of the program control meansdriven by the laser cutting strategy file to scan the at least one layerwith the respective at least one selected plurality of scans at least tofacilitate dicing of the substrate such that a resultant die has atleast a predetermined die strength and a yield of operational die equalsat least a predetermined minimum yield.

Preferably, the steps of providing a laser cutting strategy filecomprise, for each of the at least one layer, the steps of varying atleast one of a combination of pulse rut; pulse energy, pulse spatialoverlap to provide a respective combination; measuring a cutting rate ofthe respective layer using the respective combination; examining thelayer to determine whether damage is restricted to a predeterminedextent; dicing the substrate and measuring yield of the resultant die;measuring die strength of the resultant die; creating a laser cuttingstrategy file of a selected combination which maximizes cutting ratewhile resulting in a yield of operational die which have at least thepredetermined minimum yield and for which the die have at least thepredetermined die strength scanning the at least one layer using theselected combination to determine a plurality of scans necessary to cutthough the layer; and storing the selected plurality of scans in thelaser cutting strategy file.

Conveniently, the die strength is measured using a Weibull die strengthtest.

Advantageously, the step of using the laser to scan the at least onelayer includes providing a galvanometer-based scanner.

Conveniently, the step of using the laser to scan the at least one layerincludes providing a telecentric scan lens for scanning a laser beamfrom the laser across the substrate and the step of providing a lasercutting strategy file comprises the steps of: mapping a laser energydensity received in a focal plane of the telecentric scan lens toproduce a laser energy density map of a field of view of the telecentriclens using the selected combination of pulse rate, pulse energy andpulse spatial overlap of pulses; storing the laser energy density map asan array in the storage means; and using the laser energy density map tomodify, with the control means, at least one of the pulse repetitionrate and the pulse energy of the selected combination to produce aconstant laser energy density at scanned points in the field of view atthe substrate.

Conveniently, the step of mapping a laser energy density comprises usinga laser power meter to measure laser energy density at representativelocations within the field of view of the telecentric lens.

Advantageously, the step of providing a selected combination comprisesproviding a selected combination which restrict thermal loading of thematerial of the respective layer to restrict mechanical stress to apredetermined maximum.

Conveniently, the selected combination is used for less than theselected plurality of scans, which corresponds to the selectedcombination, to machine a layer to be cut and the layer is scanned forfurther scans up to the selected plurality using a combination whichwill not significantly machine an underlying layer such thatsubstantially no machining occurs of the underlying layer should thelaser continue to scan the substrate after the layer to be out has beencut through.

Advantageously, the method comprises scribing a substrate through thelayer to be cut for subsequent mechanical dicing of the substrate.

Conveniently, where the substrate includes an active layer, the step ofproviding a selected combination to restrict damage to the at least onelayer comprises providing a selected combination which does notsignificantly affect the subsequent operation of active devices in theactive layer.

Advantageously, the step of providing a selected combination which doesnot significantly affect the subsequent operation of active devices inthe active layer comprises providing a combination which does not causesignificant cracks to propagate through the active layer.

Conveniently, the step of providing a selected combination comprises thesteps of: providing an initial combination at which the laser machinesthe substrate at an initial rate which does not cause significant crackpropagation due to thermal shock at an ambient temperature, and suchthat a temperature of the substrate is raised by the machining after apredetermined plurality of scans of the substrate by the laser to araised temperature above ambient temperature; and providing a workingcombination at which the laser machines the substrate at a working rate,higher than the initial rate, which does not cause significant crackpropagation due to thermal shock at the raised temperature; and the stepof machining the substrate includes: machining an initial depth of thesubstrate using the initial combination for at least the predeterminedplurality of scans; and machining at least part of a remaining depth ofthe substrate using the working combination.

Preferably, an energy of at least a first of the plurality of scans islower than an energy of succeeding scans of the plurality of scans suchthat a generation of surface micro-cracks is less than would otherwisebe produced.

Advantageously, an energy of at least a final of the plurality of scansis lower than an energy of preceding scans of the plurality of scanssuch that backside chipping of the substrate is less than wouldotherwise be produced.

Advantageously, energy of the plurality of scans is varied between scansto facilitate removal of debris generated during dicing of thesubstrate, conveniently by increasing laser energy with increasingmachining depth to remove debris from a dice lane.

Advantageously, the method includes the further steps of: providing gashandling means to provide a gaseous environment for the substrate; usingthe gaseous environment to control a chemical reaction with thesubstrate at least one of prior to, during and after dicing thesubstrate to enhance a strength of the resultant die.

Conveniently, the step of providing gas handling means includesproviding gas delivery head means for delivering gas substantiallyuniformly to a cutting region of the substrate to facilitatesubstantially uniform cutting of the substrate.

Advantageously, the step of providing gas handling means comprisesproviding means to control at least one of flow rate, concentration,temperature, type of gas and a mixture of types of gases.

Conveniently, the step of providing a gaseous environment comprisesproviding a passive inert gas environment for substantially preventingoxidation of walls of a die during machining.

Alternatively, tie step of providing a gaseous environment comprisesproviding an active gas environment.

Conveniently, the step of providing an active gas environment comprisesetching walls of a die with the active gas to reduce surface toughnessof the walls and thereby improve the die strength.

Advantageously, the step of providing an active gas environmentcomprises etching walls of a die with the active gas substantially toremove a heat affected zone produced during machining, and therebyimprove the die strength.

Advantageously, the step of providing an active gas environmentcomprises reducing debris, produced during machining, adhering tosurfaces of machined die.

Conveniently, the method comprises the further step after dicing ofscanning an edge of the resultant die with the laser with sufficientenergy to heat sidewalls of the resultant die to reduce surfaceroughness thereof and thereby increase die strength of the resultantdie.

Conveniently, the method is adapted for producing die with roundedcorners by scanning the laser beam along a curved trajectory at cornersof the die using a galvanometer based scanner, wherein the selectedcombination is adapted to maintain the selected pulse spatial overlapbetween consecutive laser pulses around an entire circumference of thedie.

Conveniently, the selected combination is adapted to deliver pulses atan arcuate portion or corner of the die such that substantially noover-cutting or undercutting generating a defect at the arcuate die edgeor corner occurs.

Advantageously, the method is adapted for forming a tapered dice lanehaving arcuate walls tapering inwards in a direction away from the laserbeam by varying a width of the dice lane as the laser scans through thesubstrate wherein the selected combination is modified to give a finelycontrolled taper and smooth die sidewalls, and thereby increase diestrength of the resultant die.

Conveniently, the laser is a Q-switched laser device.

Preferably, a laser beam from the laser is directed by rotatablemirrors.

Conveniently, the substrate is mounted on a tape and energy of finalscans of the laser is controlled substantially to prevent damage to thetape.

Preferably, the tape is substantially transparent to ultravioletradiation.

Advantageously, the tape is polyolefin-based.

According to a second aspect of the invention, there is provided anapparatus for program-controlled dicing of a substrate comprising atleast one layer, the apparatus comprising: a pulsed laser; and programcontrol means and associated data storage means for controlling thepulsed laser using a laser cutting strategy file, stored in the datastorage means, of at least one respective selected combination of pulserate, pulse energy and pulse spatial overlap of pulses produced by thelaser at the substrate and data representative of at least onerespective selected plurality of scans of the respective at least onelayer by the pulsed laser necessary to cut through the respective atleast one layer; such that in use a resultant die has at least apredetermined die strength and a yield of operational die equals atleast a predetermined minimum yield.

Preferably, the program control means includes control means for varyingat least one of pulse rate, pulse energy and pulse spatial overlap forcontrolling the laser subject to the at least one respective selectedcombination.

Conveniently, the apparatus includes telecentric scan lens means forscanning a laser beam from the laser across the substrate.

Advantageously, the apparatus includes laser power measuring means formapping a laser energy density received in a focal plane of thetelecentric scan lens to produce a laser energy density map of a fieldof view of the telecentric lens using the selected combination of pulserate, pulse energy and pulse spatial overlap of pulse for stowing thelaser energy density map as an array in the data storage mean formodifying the at least one respective selected combination to compensatefor irregularities, introduced by the telecentric lens, of laser energydensity at the substrate.

Preferably, the apparatus flier comprises gas handling means forproviding a gaseous environment for the substrate for controlling achemical reaction with the substrate at least one of prior to, duringand after dicing the substrate to enhance strength of the resultant die.

Advantageously, the gas handling means includes gas delivery head meansfor uniformly delivering gas to a cutting region of the substrate.

Preferably, the gas handling means comprises control means forcontrolling at least one of flow rate, concentration, temperature, typeof gas and a mixture of types of gases.

Conveniently, the gas handling means is arranged to provide an inert gasenvironment for substantially preventing oxidation of walls of a dieduring machining.

Alternatively, the gas handling means is arranged to provide an activegas environment.

Advantageously, the gas handling means is arranged to etch walls of adie with the active gas to reduce surface roughness of the walls, andthereby increase die strength.

Advantageously, the gas handling means is arranged to etch walls of adie with the active gas substantially to remove a heat affected zoneproduced during machining, and thereby increase die strength.

Advantageously, the gas handling means is arranged to etch walls of adie with the active gas to reduce debris, produced during machining,adhering to surfaces of machined die.

Conveniently, the apparatus further comprises a galvanometer-basedscanner for producing die with rounded corners by a laser beam along acurved trajectory at corners of the die, wherein the selectedcombination is arranged to maintain the selected pulse spatial overlapbetween consecutive laser pulses around an entire circumference of thedie.

Advantageously, the selected combination is arranged to control laserpulse delivery at an arcuate portion or corner of a die edge such thatsubstantially no over-cutting or undercutting occurs which wouldgenerate a defect at the die edge.

Conveniently, the apparatus is arranged for forming a tapered dice lanehaving arcuate walls tapering inwards in a direction away from the laserbeam by varying a width of the dice lane as the laser scans through thesubstrate wherein the selected combination is modified to give a finelycontrolled taper with smooth die walls, and thereby increase diestrength of the resultant die.

Preferably, the laser is a Q-switched laser device.

Conveniently, the apparatus includes rotatable mirrors directing a laserbeam from the laser on the substrate.

Preferably, the apparatus is arranged for a substrate mounted on a tape,wherein the laser is controlled in final scans of the substrate notsubstantially to damage the tape.

Conveniently, the tape is substantially transparent to ultravioletlight.

Advantageously, the tape is polyolefin-based.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example onlywith reference to the accompanying drawings in which:

FIG. 1 is a plan view of a diced silicon wafer;

FIG. 2 shows transmitted laser intensity as a percentage of incidentlaser intensity over a field of view (40 mm×40 mm) of a telecentric scanlens objective for use with the invention and also the variation inlaser pulse energy for machining a trench of uniform depth according tothe invention;

FIG. 3( i) is a vertical cross-section of a multilayered substratesuitable for dicing according to the invention;

FIG. 3( ii) represents a four-step laser process used to dice themultilayered substrate in FIG. 3( i) according to the invention;

FIGS. 4( i) to 4(iii) are vertical cross-sections of a multilayeredstructure in which a pre-scribe trench is machined in a top layeraccording to the invention;

FIGS. 5( i)a and 5(i)b are vertical cross-sections of a semiconductorsubstrate diced from an active device side according to the invention;

FIGS. 5( ii)a to 5(ii)c are vertical cross-sections of a semiconductorsubstrate diced from a side opposed to the active device side accordingto the invention;

FIG. 6 is a graph, helpful in understanding the invention, of relativedie strength as abscissa and proportion of die surviving as ordinatesfor different spatial overlaps of successive laser pulses;

FIG. 7( i) shows a plurality of die with rounded corners, producedaccording to the invention;

FIG. 7( ii) shows a plurality of conventionally diced die according tothe prior art;

FIG. 8 is a schematic vertical cross-section of tapered dice lane sidewalls produced according to the invention;

FIGS. 9( i) to 9(iii) are vertical cross-sections of a single layerstructure machined, according to the invention; and

FIG. 10 is a vertical cross-section of a substrate machined according tothe invention mounted on a carrier tape.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A laser beam may be used to dice a semiconductor wafer 10 and therebysingulate devices 11 from the wafer by scanning a Q-switched laser beamover the wafer surface using rotating mirrors in a galvanometer typesystem to form a pattern such as that shown in FIG. 1. Focusing of thelaser beam may be achieved using a telecentric type scan lens.

In embodiments of this invention, the temporal separation of consecutivelaser pulses (Δt) and the laser pulse energy (E) is varied duringmachining of a single or multilayered substrate in order to reducethermal loading in different portions of the single layer or in each ofthe materials in the substrate and the subsequent mechanical stress ordamage that results.

By way of example, a multilayered material workpiece 30 consisting offour layers 31, 32, 33 and 34 of three different material types is shownin FIG. 3( i). These materials could be, for example, a polymer materialfirst layer 31 on a metal second layer 32 on a polymer third layer 33 ona semiconductor substrate 34. FIG. 3( ii), which is a plot of time(corresponding to distance machined though the multilayer s) as abscissaand pulse energy as ordinates, illustrates a four step approach todicing the substrate. In order to machine the first layer 31 in such away as to reduce thermal loading and consequent mechanical damage in thepolymer material laser pulse energy E₁ is low and an inter-pulseseparation Δt₁ is high. Polymer materials will melt and be damaged athigh laser energies of, for example several hundred microJoules perpulse, but they will be cut cleanly at lower laser pulse energies, forexample 10 microJoules per pulse. Also if the repetition rate is toohigh (i.e. Δt is too low) too much heat will enter the polymer materialover too short a time and the polymer will melt and be damaged, so forpolymers the repetition rate is kept low (i.e. Δt is high). In general,values of Δt and E are chosen based on known optical and thermalproperties of the material or are determined experimentally. The numberof laser pulses delivered at Δt₁ and E₁ is determined by the thicknessof first layer 31.

After machining through the first layer 31 with laser beam 35, the laserparameters are changed to Δt₂ and E₃, where chosen values of Δt₂ and E₃(like Δt and E for all layers in the substrate) are determined by thethermal properties and also the optical absorption properties of thematerial at the laser wavelength used. After the machining of the secondlayer 32, the laser properties are returned to Δt₁ and E₁ to machine thethird layer 33 which is of similar material to the first layer 31. Aftermachining of the third layer 33, the laser properties are changed to Δt₃and E₂ to machine the fourth layer 34. During machining of each layer inthe multilayer substrate the pulse energies E₁, E₂ and E₃ may be variedin a manner to be described across the field of view of the focusingobjective in order to compensate for irregularities in the laser energytransmitted by the telecentric lens, to ensure uniformity of machiningthrough each layer of the substrate.

In practice, prior to machining the layered substrate, a laser cuttingstrategy file is generated to contain a series of commands to the laserto change Δt and E for each layer and to control a galvanometer scannerfor positioning of the laser beam on the workpiece surface. In addition,a respective number of laser scans necessary to cut through eachrespective layer is pre-programmed in the laser cutting strategy filefrom a prior knowledge of thicknesses of each of the layers 31, 32, 33,34.

Initially, this data may be collected experimentally, by scanning layersof different materials using different pulse energies and pulserepetition rates and observing any damage, for example melting or crackpropagation in the layer. The resultant effect on die strength ofdifferent pulse energies and pulse repetition rates may also bedetermined using, for example, a known Weibull die strength test and acombination selected for each layer which produces die with at leastrequired die strength. In addition, the yield of die may be determinedto ensure that the selected combination is not damaging devices on thesubstrate and thereby adversely affecting the yield. Having selected acombination of pulse energy and pulse repetition rate which causes onlyacceptable damage and produces die of a required die strength andacceptable yield, the number of scans required to cut though a knownthickness of material may then also be determined experimentally. Thesevalues may then be used to write, the laser cutting strategy file.

Dicing in this way leads to superior die strength compared withconventional laser dicing methods.

In a further embodiment of this invention, the inter-pulse temporalseparation Δt and the laser pulse energy E are changed during themachining of a single layer of a multilayered material. Referring toFIGS. 4( i) to 4(iii), a first layer 41 to be machined with a laser beam44 overlies a second layer 42 on a substrate 43. As the first layer 41is machined, the pulse properties Δt and E of the laser beam 44 arechanged just prior to completion of machining trough the first layer 41,as illustrated by the changed broken line representing the changed laserbeam 441, during machining of the first layer 41 in order to preventdamage to an underlying second layer 42. In general, damage to theunderlying layer 42 is prevented by reducing the pulse energy E to belowa melting threshold of the material constituting the underlying layer. Atrench 45 machined in layer 41 of FIG. 4 can be used as a pre-mechanicalscribing trench. In this case die strength is improved compared with theprior art as, by appropriate choices of laser pulse energy and pulserepetition rate, there is no cracking in the top layer 41 or in theunderlying layer 42 that could grow during a mechanical scribe and breakprocess performed after the laser prescribe step.

In a further embodiment of this invention, illustrated in FIG. 5, lowenergy laser pulses of a laser beam 54 are used in a first few passesalong a dice lane 55 in order to prevent the development of large crackspropagating through active devices 51 when machining from an activedevice side of the wafers 50, as shown in FIG. 5( i)a. After the laserhas cut through a depth of material approximately equal to an activedevice layer thickness a pulse energy E of the laser beam 54 can beincreased to a higher pulse energy of a laser beam 541 under control ofa laser cutting strategy file, as shown in FIG. 5( i)b, in order tomachine more quickly the bulk of a semiconductor substrate 53 of thewafer 50, which is heated by the initial machining so that effects ofthermal shock in machining the substrate 53 are reduced. When machininginstead from a back side of a wafer substrate 53, as shown in FIG. 5(ii)a, opposed to a side caring the active devices 51, a similar processcan be adopted in order to prevent cracks propagating from the initiallaser cut down through the substrate material and so the laser beam 54with a low laser pulse energy is used initially. In the bulk of thesemiconductor substrate 53 the laser energy is increased under controlof a laser cutting strategy file using higher energy laser beam 541 forfaster machining, see FIG. 5( ii)b. When the laser beam 541 machiningfrom the backside of the wafer 50 reaches a region containing activedevices 51, the laser pulse energy of the laser beam 54 is reduced undercontrol of the laser cutting strategy file to prevent excessive damagein this region, see FIG. 5( ii)c. In order to control laser machining inthis manner, the laser cutting strategy file also contains datarepresentative of a number of scans necessary to pass through the activelayer and through the remainder of the substrate respectively, and thenumber of initial scans necessary to raise the temperature of thesubstrate to a temperature at which the effects of thermal shock areinsignificant at the raised temperature and raised pulse energy.

In a further embodiment of the invention, illustrated in FIG. 9, whenmachining, for example, a trench or dice lane 92 in, for example, asingle layer substrate 93, by multi-pass cutting, a laser beam 94 withlower pulse energy is used during an initial pass or passes Man a laserbeam 941 used when cutting a bulk of the substrate in order to prevent,or at least to reduce to a lower degree than would otherwise occur,generation of surface micro-cracks in a first surface 91 from which thesubstrate 93 is machined. Similarly, the energy of final passes of alaser beam 942 may be reduced below that used for cutting the bulk ofthe substrate 93, to prevent, or at least to reduce below a degree thanwould otherwise occur, chipping or cracking of a second surface 94 ofthe substrate opposed to the first surface 91, or, for example, at abase of a trench. In the bulk of the substrate 93 higher energy pulsesare used for efficient material removal. The pulse energy may beincreased with increasing machining depth in order to facilitate moreefficient material removal.

Moreover, referring to FIG. 10, energy of a laser beam 104 may be variedthroughout machining of a substrate 103 to facilitate removal of debris109 generated by the machining. That is, a higher peak power of thelaser beam 104 is used deep within the substrate than close to surfacesof the substrate.

The mechanical die strength of laser cut die is a function of thespatial overlap between consecutive laser pulses. The spatial overlapbetween consecutive laser pulses is therefore preferably chosen so as toyield optimum mechanical die strength of die obtained from a substrateto be machined. For example, the dependence of mechanical die strengthof a silicon substrate machined using a 355 nm Q-switched laser is shownin FIG. 6, where a probability of survival of a pressure test is plottedas ordinates against a pressure applied to a die as abscisa for a seriesof pulse overlaps from 30% to 76%. It is apparent in this case, that theplot 61 having the highest die strengths is obtained for a pulse overlapof 30%. It would appear that if the laser pulse overlap is too highthere is too much heating in a region and too much cracking. If thelaser pulse overlap is lower there is less thermal damage in a regionand less cracking. In practice, a suitable overlap to give a requireddie strength and yield may be determined experimentally and stored inthe laser cutting strategy file for use during machining. It will beunderstood that the spatial overlap of laser pulses is in fact afunction of the scanning speed the pulse repetition rate and thediameter of the incident laser beam, so that only these parameters needbe stored in the laser cutting strategy file.

When a telecentric lens is used to focus a laser beam the received laserintensity varies across a field of view of the telecentric lens. Laserparameters may be changed depending on the location of a focal spotwithin a field of view of the focusing scan lens objective in order tomaintain a constant power density at the workpiece surface across theentire field of view. The variation in transmitted laser intensity as apercentage of incident laser intensity over the field of view of atypical telecentric scan lens is shown in a contour plot 20 in an upperhalf of FIG. 2. Such a contour plot may be obtained by placing a laserpower meter beneath the telecentric lens in a plane in which thesubstrate or workpiece is to be located. Laser power readings arerecorded at a number of positions across the field of view of the lens(typically 40 mm×40 mm) and then plotted as a two dimensional surfaceplot. The irregularities in the laser power density map are mainly dueto the quality of the antireflection coating on the lenses. Atelecentric lens consists of a number of lenses and any irregularitiesin thickness or quality of the coating on any of these lenses can causethe observed irregularities in the laser power density map. Also, due tothe geometry of a telecentric lens, its inherent performance is not sogood at the edges of the field of view so the laser power density isreduced because of distortion in the laser beam profile caused by thetelecentric lens itself.

Maintaining a constant power density across the entire scan lens fieldof view necessitates changing at least one of laser pulse energy andlaser repetition frequency. In this embodiment of the invention, laserparameters are changed depending on the location of a focal spot withinthe field of view of the focusing objective in order to maintain aconstant power density at the workpiece surface across the entire fieldof view. The variation in transmitted laser intensity as a percentage ofincident laser intensity over the field of view of a typical telecentricscan lens is shown in FIG. 2. Maintaining a constant power densityacross the entire scan lens field of view necessitates changing at leastone of the laser pulse energy and the laser repetition rate andconveniently changing the laser pulse energy at a fixed laser repetitionfrequency or, alternatively, changing the laser repetition frequency ata fixed laser pulse energy. Power density (φ) is defined as the power (Pin units of Watts) per unit area (A in units of centimetres squared) atthe focal spot of the laser and is given by

$\phi = \frac{P}{A}$

Where the power equals the pulse energy (E in units of Joules) persecond (s)

$P = \frac{E}{s}$

By way of example, a lower half of FIG. 2, which is a plot of pulseenergy as ordinates versus distance along the line 21, which is 10 mmfrom the lower edge of the field as seen in the upper half of FIG. 2,demonstrates the modification of laser pulse energy that is required tomaintain a constant power density at the substrate while scanning thelaser across the field of view of the scan lens to compensate for thevariation in transmitted laser intensity by the telecentric lens. Inthis example, the laser is scanned along straight line 21 which is 40 mmin length, 10 mm from the centre of the lens. In the upper half of FIG.2 the field of view of the lens is divided into regions wherein theintensity at each point in a given region is within .+−0.5% of allpoints in that region. For the 40 mm line 21 scanned by the laser inthis example, six different regions, corresponding to six portions 22,23, 24, 25, 26, 27 of the scan line 21, are traversed and as a resultthe laser energy is changed, under the control of the laser cuttingstrategy file, five times. The laser pulse energy starts at value 221 ofE₄ in region 1 for a first portion 22 of scan line 21. The transmittedlaser intensity at the workpiece in region 1 is 80 to 85% of the laserintensity incident on the scan lens and as region 1 represents theregion of lowest incident laser intensity compared to all the regions 2to 6, the energy per laser pulse E₄ in region 1 is consequently thehighest. As the laser is scanned from region 1 to region 2,corresponding to a second portion 23 of scan line 21, the transmittedlaser intensity increases to 85 to 90% of the laser intensity incidenton the scan lens and in order to maintain a constant power density onthe surface of the workpiece the laser pulse energy is now reduced to avalue 231 of E₃, where E₃ is 5% lower in energy than E₄. As the laserbeam traverses from one region to the next the laser pulse energy ischanged, under control of the laser cutting strategy file, ‘on the fly’(on a pulse to pulse basis if required) in order to maintain a constantvalue of power density (φ) at the workpiece surface along the entire 40mm length of the dice lane 21.

In summary, the laser power density φ at the workpiece surface isdirectly proportional to the laser pulse energy E. The value of thelaser pulse energy at the workpiece surface will differ from thatemerging directly from the laser due to attenuation in the scan lens.The contour map is stored as a two dimensional array in a computermemory associated with a computer control of the laser and depending onwhere the software directs the galvanometer scanner to place the laserbeam in the field of view, a simultaneous command is sent to the laserto change the pulse repetition rate and the laser pulse energy asindicated in the laser cutting strategy file. Laser power may also bemonitored by an integral power meter in the laser head itself and anyvariation in power in the laser can be compensated for. In principle,rather than storing the contour map the laser power could be monitoredat the workpiece or substrate but there would be a loss of laser powerin doing so, and preferably the contour map is stored in memory. Inaccordance with the invention, the combination of pulse repetition rateand pulse energy are controlled during scanning, the laser pulse energyE is varied in indirect proportion to the transmission of thetelecentric scan lens, in order to maintain a constant power density atthe workpiece surface across the entire field of view of the scan lens.This permits, for example, the machining of dice lanes and pre-scribingtrenches of uniform depth, where the depth of the dice lane is directlyproportional to the power density cp. In instances where a substrate islaser machined so that the laser cuts down through the entire thicknessof the substrate, maintaining uniform power density across the entiredice lane prevents partial cutting of dice lanes. Partial cutting ofdice lanes leaves material between adjacent die and during the pick andplace process, when die are picked from a transport tape, such die whichare stuck together may break apart causing damage to the die, thusreducing significantly their mechanical strength.

Laser dicing in accordance with the invention may be performed in anon-ambient gas environment controlled by a gas handling system. Gasparameters such as S flow rate, concentration, temperature, gas type andgas mixes may be controlled at least one of prior to, during and afterthe laser dicing process. A series of different gases may be used atleast one of prior to, during and after the laser machining processes.

A gas delivery head may be used to ensure gas is uniformly delivered toa cutting region of the substrate such that uniform cutting is achieved.

Gases used may be passive or reactive with respect to the semiconductorsubstrate and/or layers in the semiconductor wafer or substrate. Inertgases (e.g. argon and helium) may be used to prevent growth of an oxidelayer on the die walls during laser machining. Gases that react withsilicon (e.g. chlorofluorocarbons and halocarbons) may be used at leastone of prior to, during and after laser machining to reduce the surfaceroughness of die sidewalls by etching the substrate material. Also, aheat affected zone (HAZ) produced on the die sidewalls as a result oflaser machining can be etched away using a reactant gas. In this way thequality of the die sidewalls is improved and therefore the die strengthis increased. Also reactive gases reduce the amount of debris adheringon die sidewalls and top and bottom surfaces, thus reducing potentialstress points on laser machined die.

In a further embodiment of this invention, the laser pulse energy isreduced to a value close to the melting threshold of the wafer material(after die singulation) and the laser is scanned along the die edge inorder to heat (rather than ablate) the die sidewalls. In doing so, thesurface roughness of the die sidewalls is reduced and the uniformity ofthe heat affected zone is increased, thus resulting in increased diestrength.

In a further embodiment of his invention, the laser is scanned in such away as to machine die 71 with rounded corners 72 as shown in FIG. 7( i).Die 75 diced with a conventional mechanical saw according to the priorart are shown in FIG. 7( ii). Rounded corner geometry is easier toachieve and is more accurate when using a galvanometer based lasermachining system rather than a conventional mechanical saw based dicingsystem. However, the laser pulse properties must be changed at therounded corner sections if as is typically the case, the galvanometerscanning mirrors used to direct the laser beam have to slow down as theytraverse the curved features. Otherwise, when the scanning mirrors slowdown the laser pulse spatial overlap would increase, therefore the timebetween pulses, Δt, needs to be increased in order to maintain anoverlap on the rounded corner sections that is the same as a spatialoverlap used on sight regions of the die. This data is stored in thelaser cutting strategy file for controlling the laser beam duringmachining. Using a laser to produce die with rounded corners improvesdie strength and enables dicing of thin wafers. The rounded cornerseliminate stresses that are induced by sharp corners of rectangular die.

In addition, machining may be controlled by the laser cutting strategyfile and program control such that pulse delivery on a corner or curvedportion of a die edge is such that a “clear” corner or curved section isobtained with no over-cutting or undercutting which may otherwisegenerate a defect at the die edge.

In a further embodiment of the invention, the taper of a laser dice lane85, cut with a laser beam 84 in a substrate 83, may be varied in orderto produce convex arcuate die sidewalls 82, as shown in FIG. 8, toproduce a cut which tapers in a direction of the laser beam 84. As inthe previously described embodiment, this results in increased diestrength by removing potential stress points at sharp corners. Taperingof the dice lane sidewalls is achieved by varying the width of the dicelane as the laser beam scans down through the substrate. The taperedsidewalls shown in FIG. 8 are achieved by reducing the number ofadjacent laser scans in the dice lane as the depth machined into thesubstrate increases.

As illustrated in FIG. 10, a substrate 103 to be machined may be mountedon a transport tape 110, for example to singulate die 101 by machiningdice lanes 102 in the substrate 103 in that case, the laser beam energymay be controlled in final passes through the substrate to ensure thatdamage to the tapes does not occur, as described above in relation toFIG. 9( iii). Alternatively, or in addition, a tape 110 may be used,such as a polyolefin-based tape, which is substantially transparent toan ultraviolet laser light beam 104 used to machine the substrate 103,such that, with suitable choices of machining process parameters,substantially no damage occurs to the tape.

The invention is not limited to the embodiments described but may bevaried in construction and detail.

1. A method of using a pulsed laser for program-controlled dicing of asubstrate comprising at least one layer, the method comprising the stepsof: a. providing program control means and associated data storage meansfor controlling the pulsed laser; b. providing in the associated datastorage means a laser cutting strategy file of at least one selectedcombination of pulse rate, pulse energy and pulse spatial overlap ofpulses produced by the laser at the substrate to restrict damage to therespective at least one layer while maximising machining rate for the atleast one layer; c. providing in the laser cutting strategy file datarepresentative of at least one selected plurality of scans of therespective at least one layer by the pulsed laser necessary to cutthrough the respective at least one layer when the pulsed laser isoperating according to the respective at least one combination stored inthe laser cutting strategy file; and d. using the laser under control ofthe program control means driven by the laser cutting strategy file toscan the at least one layer with the respective at least one selectedplurality of scans at least to facilitate dicing of the substrate suchthat a resultant die has at least a predetermined die strength and ayield of operational die equals at least a predetermined minimum yield.2. A method as claimed in claim 1, wherein the steps b and c ofproviding a laser cutting strategy file comprise, for each of the atleast one layer, the steps of: b1. varying at least one of a combinationof pulse rate, pulse energy, pulse spatial overlap to provide arespective combination; b2. measuring a cutting rate of the respectivelayer using the respective combination; b3. examining the layer todetermine whether damage is restricted to a predetermined extent; b4.dicing the substrate and measuring yield of the resultant die; b5.measuring die strength of the resultant die; b6. creating a lasercutting strategy file of a selected combination which maximises cuttingrate while resulting in a yield of operational die which have at leastthe predetermined minimum yield and for which the die have at least thepredetermined die strength; c1. scanning the at least one layer usingthe selected combination to determine a plurality of scans necessary tocut through the layer; and c2. storing the selected plurality of scansin the laser cutting strategy file.
 3. A method as claimed in claim 1,wherein the selected combination is used for less than the selectedplurality of scans, which corresponds to the selected combination, tomachine a layer to be cut and the layer is scanned for further scans upto the selected plurality using a combination which will notsignificantly machine an underlying layer such that substantially nomachining occurs of the underlying layer should the laser continue toscan the substrate after the layer to be cut has been cut through.
 4. Amethod as claimed in claim 1, wherein the substrate includes an activelayer, wherein the step of providing a selected combination to restrictdamage to the at least one layer comprises providing a selectedcombination which does not significantly affect the subsequent operationof active devices in the active layer.
 5. A method as claimed in claim1, wherein the step of providing a selected combination comprises thesteps of: b7. providing an initial combination at which the lasermachines the substrate at an initial rate which does not causesignificant crack propagation due to thermal shock at an ambienttemperature, and such that a temperature of the substrate is raised bythe machining after a predetermined plurality of scans of the substrateby the laser to a raised temperature above ambient temperature; b8. andproviding a working combination at which the laser machines thesubstrate at a working rate, higher than the initial rate, which doesnot cause significant crack propagation due to thermal shock at theraised temperature; and step d of machining the substrate includes: d4.machining an initial depth of the substrate using the initialcombination for at least the predetermined plurality of scans; and d5.machining at least part of a remaining depth of the substrate using theworking combination.
 6. A method as claimed in claim 1, including thefurther steps of: e. providing gas handling means to provide a gaseousenvironment for the substrate; f. using the gaseous environment tocontrol a chemical reaction with the substrate at least one of prior toand during dicing the substrate to enhance a strength of the resultantdie.
 7. A method as claimed in claim 6, wherein the step of providing agaseous environment comprises providing a passive inert gas environmentfor substantially preventing oxidation of walls of a die duringmachining.
 8. A method as claimed in claim 6, wherein the step ofproviding a gaseous environment comprises providing an active gasenvironment.
 9. A method as claimed in claim 8 wherein the step ofproviding an active gas environment comprises etching walls of a diewith the active gas to reduce surface roughness of the walls and therebyimprove the die strength.
 10. A method as claimed in claim 8, whereinthe step of providing an active gas environment comprises etching wallsof a die with the active gas substantially to remove a heat affectedzone produced during machining, and thereby improve the die strength.11. A method as claimed in claim 8, wherein the step of providing anactive gas environment comprises reducing debris, produced duringmachining, adhering to surfaces of machined die.
 12. A method as claimedin claim 1, for producing die with rounded corners by scanning the laserbeam along a curved trajectory at corners of the die using agalvanometer based scanner, wherein the selected combination is adaptedto maintain the selected pulse spatial overlap between consecutive laserpulses around an entire circumference of the die.
 13. A method asclaimed in claim 1, wherein the selected combination is adapted todeliver pulses at an arcuate portion or corner of the die such thatsubstantially no over-cutting or undercutting generating a defect at thearcuate die edge or corner occurs.
 14. A method as claimed in claim 1,to form a tapered dice lane having arcuate walls tapering inwards in adirection away from the laser beam by varying a width of the dice laneas the laser scans through the substrate wherein the selectedcombination is modified to give a finely controlled taper and smooth diesidewalls, and thereby increase die strength of the resultant die.
 15. Amethod as claimed in claim 1, wherein the substrate is mounted on a tapeand energy of final scans of the laser is controlled substantially toprevent damage to the tape.
 16. A program-controlled substrate dicingapparatus for dicing a substrate comprising at least one layer, theapparatus comprising: a pulsed laser; program control means andassociated data storage means for controlling the pulsed laser using alaser cutting strategy file, stored in the data storage means, of atleast one respective selected combination of pulse rate, pulse energyand pulse spatial overlap of pulses produced by the laser at thesubstrate and data representative of at least one respective selectedplurality of scans of the respective at least one layer by the pulsedlaser necessary to cut through the respective at least one layer;telecentric scan lens means for scanning a laser beam from the pulsedlaser across the substrate; and laser power measuring means for mappinga laser energy density received in a focal plane of the telecentric scanlens to produce a laser energy density map of a field of view of thetelecentric lens using the selected combination of pulse rate, pulseenergy and pulse spatial overlap of pulses for storing the laser energydensity map as an array in the data storage means for modifying the atleast one respective selected combination to compensate forirregularities, introduced by the telecentric lens, of laser energydensity at the substrate, such that in use a resultant die has at leasta predetermined die strength and a yield of operational die equals atleast a predetermined minimum yield.
 17. An apparatus as claimed inclaim 16, wherein the program control means includes control means forvarying at least one of pulse rate, pulse energy and pulse spatialoverlap for controlling the laser subject to the at least one respectiveselected combination.
 18. An apparatus as claimed in claim 16, furthercomprising gas handling means for providing a gaseous environment forthe substrate for controlling a chemical reaction with the substrate atleast one of prior to and during dicing the substrate to enhancestrength of the resultant die.
 19. An apparatus as claimed in claim 16,further comprising a galvanometer-based scanner for producing die withrounded corners by scanning a laser beam along a curved trajectory atcorners of the die, wherein the selected combination is arranged tomaintain the selected pulse spatial overlap between consecutive laserpulses around an entire circumference of the die.
 20. An apparatus asclaimed in claim 16, arranged for forming a tapered dice lane havingarcuate walls tapering inwards in a direction away from the laser beamby varying a width of the dice lane as the laser scans through thesubstrate wherein the selected combination is modified to give a finelycontrolled taper with smooth die walls, and thereby increase diestrength of the resultant die.